Segregation and foraging ecology of whale sharks, Rhincodon typus, in the southwestern Gulf of California

Sharks segregate by sex and size, but few studies have attempted to explain such behaviors. To address this, we examined aggregations and the foraging ecology of whale sharks in Bahía de La Paz (BLP) with aerial and ship surveys and direct observation. Zooplankton abundance and composition, and hydrographic conditions were analyzed in relation to whale shark occurrence to explore underlying factors causing segregations. We observed large aggregations of juveniles (<9 m total length, TL) inshore, comprised by 60 % male individuals, and small aggregations of adults (>9 m TL) offshore, composed of 84 % females. Juvenile sharks were associated to turbid shallow waters in BLP, where they performed stationary and dynamic suction feeding on dense copepod swarms. Adults occurred in oceanic waters and fed by ram-filtering on diffuse patches of euphausiids, with no association to oceanographic conditions. Such segregation may be advantageous to juvenile R. typus utilizing shallow coastal waters to find abundant preferred prey needed for their fast growth rates. Our studies suggest that the main driving forces of whale shark segregation by sex and size in BLP may be diet preference for juveniles and habitat preference for adult sharks.     

Movement Patterns of Juvenile Whale Sharks Tagged at an Aggregation Site in the Red Sea

Conservation efforts aimed at the whale shark, Rhincodon typus, remain limited by a lack of basic information on most aspects of its ecology, including global population structure, population sizes and movement patterns. Here we report on the movements of 47 Red Sea whale sharks fitted with three types of satellite transmitting tags from 2009–2011. Most of these sharks were tagged at a single aggregation site near Al-Lith, on the central coast of the Saudi Arabian Red Sea. Individuals encountered at this site were all juveniles based on size estimates ranging from 2.5–7 m total length with a sex ratio of approximately 1∶1. All other known aggregation sites for juvenile whale sharks are dominated by males. Results from tagging efforts showed that most individuals remained in the southern Red Sea and that some sharks returned to the same location in subsequent years. Diving data were recorded by 37 tags, revealing frequent deep dives to at least 500 m and as deep as 1360 m. The unique temperature-depth profiles of the Red Sea confirmed that several whale sharks moved out of the Red Sea while tagged. The wide-ranging horizontal movements of these individuals highlight the need for multinational, cooperative efforts to conserve R. typus populations in the Red Sea and Indian Ocean.     

Recent strandings and sightings of whale sharks in South Africa

Available information on whale shark strandings around the coast of South Africa during the period 1984–1995 was collated. Stranded animals ranged in size from 3–11 m TL, most were immature and the sex ratio was even. Aerial observations and sightings by divers indicate that whale sharks are most abundant in South African waters during the austral summer and autumn months. This revised version was published online in July 2006 with corrections to the Cover Date.     

Whale Shark (Rhincodon typus) Seasonal Presence,Residence Time and Habitat Use at Darwin Island,Galapagos Marine Reserve

The life history of the whale shark (Rhincodon typus), including its reproductive ecology, still remains largely unknown. Here, we present results from the first whale shark population study around Darwin Island, Galapagos Marine Reserve. Following a diversified approach we characterized seasonal occurrence, population structure and size, and described habitat use of whale sharks based on fine scale movements around the island. Whale shark presence at Darwin Island was negatively correlated with Sea Surface Temperature (SST), with highest abundance corresponding to a cool season between July and December over six years of monitoring. From 2011 to 2013 we photo-identified 82 whale sharks ranging from 4 to 13.1 m Total Length (TL). Size distribution was bimodal, with a great majority (91.5%) of adult female individuals averaging 11.35 m±0.12 m (TL±SE), all but one showing signs of a potential pregnancy. Population dynamics models for apparently pregnant sharks estimated the presence of 3.76±0.90 (mean ± SE) sharks in the study area per day with an individual residence time of 2.09±0.51 (mean ± SE) days. Movement patterns analysis of four apparently pregnant individuals tracked with acoustic tags at Darwin Island revealed an intense use of Darwin''s Arch, where no feeding or specific behavior has been recorded, together with periodic excursions around the island''s vicinity. Sharks showed a preference for intermediate depths (20–30 m) with occasional dives mostly to mid-water, remaining the majority of their time at water temperatures between 24–25°C. All of our results point to Darwin Island as an important stopover in a migration, possibly with reproductive purposes, rather than an aggregation site. Current studies carried out in this area to investigate regional scale movement patterns may provide essential information about possible pupping grounds for this enigmatic species.     

Catch composition and aspects of the biology of sharks caught by Thai commercial fisheries in the Andaman Sea

Catch composition, landing patterns and biological aspects of sharks caught by commercial fishing fleet operating in the Andaman Sea were recorded from landing sites in Ranong province of Thailand over a period of 1 year. Of the 64 species previously reported in the existing Thailand checklist, only 17 species were recorded in this study. Shark landings from the Andaman Sea appear now to be dominated largely by bamboo sharks Chiloscyllium spp. (Hemiscylliidae), which contribute c. 65% of the total number of sharks recorded. The carcharhinid sharks comprised c. 30·5% to the total catch, while the remaining c. 4·5% of landings comprised sharks from the families Squalidae, Stegostomatidae, Sphyrnidae and Triakidae. The catch composition is remarkably different from the previous landing survey in 2004, in that the current study found noticeable declines in landings of slow‐growing, late‐ maturing and low‐fecundity species (especially sphyrnid and carcharhinid species). The absences of many species and changes in life‐stage composition suggest that the populations of these groups may be close to collapse. The results from this study emphasize the urgency for additional research and monitoring efforts and also the need for management incentives in order to manage shark fisheries effectively in the Andaman Sea.     

Juvenile Greenland sharks <Emphasis Type="Italic">Somniosus microcephalus</Emphasis> (Bloch &amp; Schneider, 1801) in the Canadian Arctic

Life-stage-based management of marine fishes requires information on juvenile habitat preferences to ensure sustainable population demographics. This is especially important in the Arctic region given very little is known about the life histories of many native species, yet exploitation by developing commercial and artisanal fisheries is increasing as the ice extent decreases. Through scientific surveys and bycatch data from gillnet fisheries, we document captures of rarely reported juvenile Greenland sharks (Somniosus microcephalus; ≤200 cm total length [TL]) during the ice-free period in the Canadian Arctic. A total of 22 juvenile animals (42 % of total catch; n = 54), including the smallest reliably measured individual of 117 cm TL, were caught on scientific longlines and bottom trawls in Scott Inlet and Sam Ford Trough over three consecutive years. Molecular genetic nuclear markers confirmed species identity for 44 of these sharks sampled; however, two sharks including a juvenile of 150 cm TL were identified as carrying a Pacific sleeper shark (Somniosus pacificus) mitochondrial cytochrome b (cyt b) haplotype. This represents the first record of a Pacific sleeper shark genetic signature in Greenland sharks in Eastern Arctic waters. Juvenile sharks caught as bycatch in gillnet fisheries were only observed offshore in Baffin Bay surrounding a fishery closure area, while larger subadult and mature Greenland sharks (>200 cm TL) were caught in all fishing locations, including areas where juveniles were observed. The repeatable occurrence of juvenile Greenland sharks in a fjord and their presence at two offshore sites indicates that these smaller animals either reside in nurseries or have defined home ranges in both coastal and offshore regions or undertake large-scale inshore–offshore movements.     

Regional movements of satellite‐tagged whale sharks Rhincodon typus in the Gulf of Aden

To gain insight into whale shark (Rhincodon typus) movement patterns in the Western Indian Ocean, we deployed eight pop‐up satellite tags at an aggregation site in the Arta Bay region of the Gulf of Tadjoura, Djibouti in the winter months of 2012, 2016, and 2017. Tags revealed movements ranging from local‐scale around the Djibouti aggregation site, regional movements along the coastline of Somaliland, movements north into the Red Sea, and a large‐scale (>1,000 km) movement to the east coast of Somalia, outside of the Gulf of Aden. Vertical movement data revealed high occupation of the top ten meters of the water column, diel vertical movement patterns, and deep diving behavior. Long‐distance movements recorded both here and in previous studies suggest that connectivity between the whale sharks tagged at the Djibouti aggregation and other documented aggregations in the region are likely within annual timeframes. In addition, wide‐ranging movements through multiple nations, as well as the high use of surface waters recorded, likely exposes whale sharks in this region to several anthropogenic threats, including targeted and bycatch fisheries and ship‐strikes. Area‐based management approaches focusing on seasonal hotspots offer a way forward in the conservation of whale sharks in the Western Indian Ocean.     

Multi-Year Impacts of Ecotourism on Whale Shark (Rhincodon typus) Visitation at Ningaloo Reef,Western Australia

In-water viewing of sharks by tourists has become a popular and lucrative industry. There is some concern that interactions with tourists with ecotourism operations might harm sharks through disruption of behaviours. Here, we analysed five years of whale shark (Rhincodon typus) encounter data by an ecotourism industry at Ningaloo Reef, Western Australia, to assess the impact of ecotourism interactions on shark visitation, within the context of the biological and physical oceanography of the region. Our data base consisted of 2823 encounter records for 951 individual whale sharks collected by ecotourism operators between 2007 and 2011. We found that total encounters per whale shark and encounters per boat trip increased through time. On average, whale sharks re-encountered in subsequent years were encountered earlier, stayed longer and tended to be encountered more often within a season than sharks that were only encountered in a single year. Sequential comparisons between years did not show any patterns consistent with disturbance and the rate of departure of whale sharks from the aggregation was negatively correlated to the number of operator trips. Overall, our analysis of this multi-year data base found no evidence that interactions with tourists affected the likelihood of whale shark re-encounters and that instead, physical and biological environmental factors had a far greater influence on whale shark visitation rates. Our approach provides a template for assessing the effects of ecotourism interactions and environmental factors on the visitation patterns of marine megafauna over multiple years.     

Scarring patterns and relative mortality rates of Indian Ocean whale sharks

This study recorded the scarring rate and severity for whale sharks Rhincodon typus from three Indian Ocean aggregations (Australia, Seychelles and Mozambique), and examined whether scarring (mostly attributed to boat strikes and predator attacks) influences apparent survival rates using photo‐identification libraries. Identifications were based on spot‐and‐stripe patterns that are unique to individual whale sharks. Scarring was most prevalent in the Seychelles aggregation (67% of individuals). Predator bites were the most frequent source of scarring (aside from minor nicks and abrasions) and 27% of individuals had scars consistent with predator attacks. A similar proportion of whale sharks had blunt trauma, laceration and amputation scars, the majority of which appeared to be caused by ship collisions. Predator bites were more common (44% of individuals) and scars from ship collisions were less common at Ningaloo Reef than at the other two locations (probability of among‐site differences occurring randomly = 0·0007 based on a randomized multinomial contingency analysis). In all aggregations, scars occurred most often on the caudal fin, which may result from the fin being the body part closest to the surface when boats pass over, or they may provide a large target for predator attack. No evidence was found for an effect of scarring on apparent survival (φ; mean ±s .e .) for the Ningaloo (not scarred φ= 0·858 ± 0·033; scarred φ= 0·929 ± 0·033) or Seychelles populations (not scarred φ= 0·502 ± 0·060; scarred φ= 0·538 ± 0·070). The lower apparent survival of the Seychelles population may be attributed to a high number of transient whale sharks in this aggregation that might bias estimates. This study indicates that while scarring from natural predators and smaller vessels appears to be unrelated to whale shark survival, the effect of deaths related to ship strike need to be quantified to assist in future management of this species.     

To go or not to go with the flow: Environmental influences on whale shark movement patterns

Seven whale sharks were tracked using satellite-linked tags from Ningaloo Reef, off northern Western Australia, following tagging in April and June 2002 and April-May 2005. We investigated how the movements of those whale shark tracks were influenced by geostrophic surface currents during sequential one-week periods by using a passive diffusion model parameterised with observed starting locations of the sharks and weekly maps of surface current velocity and direction (derived from altimetry). We compared the outputs from the passive diffusion model and maps of chlorophyll-a concentration (SeaWiFs/MODIS) and with the actual tracks of the sharks using GIS and generalized linear mixed-effects models (GLMM). The GLMM indicated very little support for passive diffusion with sea-surface ocean currents influencing whale shark distributions in the north eastern Indian Ocean. Moreover, the sharks' movements correlated only weakly with the spatial distribution of sea-surface chlorophyll-a concentrations. The seven whale sharks had average swimming speeds comparable with those recorded in other satellite tracking studies of this species. Swimming speeds of the seven sharks were similar to those reported in previous studies and up to three times greater than the maximum sea-surface current velocities that the sharks encountered while traversing into lower southerly latitudes (moving northward towards the equator). Our results indicate that whale sharks departing from Ningaloo travel actively and independently of near-surface currents where they spend most of their time despite additional metabolic costs of this behaviour.     

Donsol: an important reproductive habitat for the world's largest fish Rhincodon typus?

Little is known about the reproductive ecology of the whale shark Rhincodon typus. On 15 March 2020, a free-swimming neonate whale shark was found in the shallow, coastal waters of Donsol, Philippines. The total length of the male shark was 60 cm, falling within the species known size-at-birth. This is the third occurrence of neonatal whale sharks reported in the area, and coupled with the occurrence of juveniles and adults, and anecdotal mating and precopulatory behaviour reported herein, the importance of the site for this endangered species is highlighted.     

Trends in sightings and population structure of white sharks, <Emphasis Type="Italic">Carcharodon carcharias,</Emphasis> at Seal Island,False Bay,South Africa,and the emigration of subadult female sharks approaching maturity

A long-term sightings and photographic identification (photo-ID) database documented the inter-annual and monthly trends in white shark (Carcharodon carcharias) sightings, sex ratios and life-history stages at Seal Island, False Bay, South Africa, over the period 2004–2012. A total of 1105 sightings were recorded during 171 scientific surveys incorporating 577 h of observation (annual mean 64.10 h, range 23.71–178.69 h). The mean annual sighting rate was 1.87 (range 0.90–3.19) sharks per hour and sighting rates declined over the nine-year study period. It is unknown whether the decline resulted from an actual population decline, or was due to changes in shark distribution or environmental conditions, but it highlights the need for ongoing monitoring at this site. The overall sex ratio was 1.0: 1.0: 2.3 for male: female: unsexed sharks, respectively. Of the 1105 sightings, 39% (n = 433) were photo-ID’d, representing 303 individual sharks (112: 111: 80, male: female: unsexed) and 130 resightings. Of the 303 photo-ID’d sharks, 71% were sighted in a single year only, indicative of transient behaviour. Of the 29% of sharks that were resighted in more than one year, 65% were resighted in the following year, indicating site fidelity to Seal Island, whereas 35% skipped one or more years between encounters, indicative of a temporary absence. The majority (60%) of the photo-ID’d sharks were immature, 32% were subadult and only 8% were mature. No young-of-the-year sharks and few adults were recorded, indicating that Seal Island is not an adult aggregation site, nor a pupping or nursery area, but rather is best described as a seasonal feeding ground. Large females were rarely resighted again after they approached length-at-maturity (≥450 cm TL), whereas some adult males were recorded consistently across years. The emigration of subadult female sharks approaching maturity from Seal Island, combined with the small number of mature sharks of both sexes reported from any South African location, indicate that adult aggregation sites, and thus areas of reproductive importance, still remain unknown.     

Deep-diving behaviour of a whale shark Rhincodon typus during long-distance movement in the western Indian Ocean

A whale shark Rhincodon typus satellite tagged off the coast of Mozambique showed a highly directional movement across the Mozambique Channel and around the southern tip of Madagascar, a minimum distance of 1200 km in 87 days. Dives to depths well into the mesopelagic and bathypelagic zones (1286 m maximum depth) were recorded in a bathymetrically non-constraining habitat. The water temperature range recorded during the fish's movement was 3·4–29·9° C.     

Feeding habits of the whale shark (<Emphasis Type="Italic">Rhincodon typus</Emphasis>) inferred by fatty acid profiles in the northern Mexican Caribbean

Whale shark (Rhincodon typus, Smith, 1828) is an endangered species with anthropogenic pressures due to increasing demand of encounter tourism activities. Research efforts to identify management and conservation strategies for this species are needed. The Northern Mexican Caribbean is one of the most important feeding aggregation sites of whale sharks worldwide. In this study, Mexican Caribbean whale shark feeding habits are assessed by means of fatty acid (FA) signature analysis, a biochemical non-destructive technique widely applied in trophic ecology studies. Sub-dermal tissue biopsies of 68 whale sharks and samples of their potential prey (zooplankton) were collected during 2010 and 2011 in two areas with high R. typus abundance. Zooplankton samples (n?=?17) were divided in two categories: mixed zooplankton (several groups of zooplankton) and fish eggs (> 95% of sample components were fish eggs). FA profiles of whale shark tissue sampled between years showed significant variability; while there was no intraspecific differences in FA signature related to sex, size and location. FA profiles of whale sharks and their potential prey were dominated by saturated fatty acids (SFA). R. typus FA signature was significantly different from that of mixed zooplankton; on the other hand, whale shark and fish egg FA profiles formed groups with overlapping values and registered high levels of oleic acid. PUFA average ω3/ ω6 ratio on whale shark FA profiles for both years was below 1. Arachidonic acid (ARA) percentage was higher in whale shark biopsies (13.2% in 2010, 6.8% in 2011) compared to values observed in fish eggs (2.0%) and mixed zooplankton (1.4%). Similarity between FA profiles of whale sharks and fish eggs, low levels of bacterial FA found in R. typus biopsies, as well as whale shark feeding behavior observations in the study area, suggest that R. typus is feeding mainly on surface zooplankton in Mexican Caribbean; however, elevated ARA percentages in whale shark samples may indicate that this species has complementary feeding sources, such as demersal zooplankton, which has been reported in other aggregation sites. Results obtained contribute to the knowledge of the whale shark trophic ecology in the area, but are inconclusive. Further studies are recommended to evaluate whale shark FA profiles from different tissues (muscle or blood); also, broader information is needed about zooplankton FA signature in the study area.     

A streamlined DNA tool for global identification of heavily exploited coastal shark species (genus Rhizoprionodon)

Obtaining accurate species-specific landings data is an essential step toward achieving sustainable shark fisheries. Globally distributed sharpnose sharks (genus Rhizoprionodon) exhibit life-history characteristics (rapid growth, early maturity, annual reproduction) that suggests that they could be fished in a sustainable manner assuming an investment in monitoring, assessment and careful management. However, obtaining species-specific landings data for sharpnose sharks is problematic because they are morphologically very similar to one another. Moreover, sharpnose sharks may also be confused with other small sharks (either small species or juveniles of large species) once they are processed (i.e., the head and fins are removed). Here we present a highly streamlined molecular genetics approach based on seven species-specific PCR primers in a multiplex format that can simultaneously discriminate body parts from the seven described sharpnose shark species commonly occurring in coastal fisheries worldwide. The species-specific primers are based on nucleotide sequence differences among species in the nuclear ribosomal internal transcribed spacer 2 locus (ITS2). This approach also distinguishes sharpnose sharks from a wide range of other sharks (52 species) and can therefore assist in the regulation of coastal shark fisheries around the world.     

Aspects of maturation and reproduction in hexanchiform and squaliform sharks

Three species of hexanchiform sharks belonging to two families and 12 species of squaliform sharks belonging to three families were recorded in fish landing site surveys in eastern Indonesia. Of these, the Squalidae were the most abundant species landed, with Squalus hemipinnis, Squalus edmundsi and Squalus montalbani contributing 0·4, 0·4 and 0·5% to the total number of sharks recorded in a 5 year survey of Indonesian fish landing sites. In comparison, the hexanchid Hexanchus griseus contributed the largest percentage to the total shark biomass. For many species, the majority of the catch consisted of immature fishes, which had not yet been able to reproduce. The data presented in this article are the first biological data reported on most of these shark species and are thus vital for fisheries managers and conservation assessors.     

New insights into whale shark <Emphasis Type="Italic">Rhincodon typus</Emphasis> diet in Brazil: an observation of ram filter-feeding on crab larvae and analysis of stomach contents from the first stranding in Bahia state

We describe the first record of a whale shark, Rhincodon typus, feeding in Brazilian coastal waters, and the first stranding record in the state of Bahia, Northeast Brazil. In April 2008, an individual of R. typus was observed surface feeding on Dromiidae crab larvae in the continental shelf off the coast of Bahia, near a gas platform. Other fishes were observed foraging in association with the whale shark. We also document the first stranding of R. typus on the coast of Bahia in October 2013. Biometric data confirmed that the stranded whale shark was a juvenile. Stomach content analysis revealed the ingestion of Geryonidae crab larvae. Plastic debris were also found in the gastric lumen of the stranded juvenile whale shark, and we speculate that it could have been a contributing factor to the stranding, and subsequent death of the whale shark. Crab larvae were observed in both of our records and likely to consist as relevant prey items for R. typus in Brazilian continental shelf. Our study provided a contribution on the diet and feeding behaviour of whale sharks in tropical oligotrophic waters and highlights the risks of marine pollution for the species conservation.     

Aggregations of juvenile whale sharks (<Emphasis Type="Italic">Rhincodon typus</Emphasis>) in the Gulf of Tadjoura,Djibouti

A total of 23 whale sharks were identified over a 5 d period in the Arta Bay region of the Gulf of Tadjora, Djibouti. Most of the sharks aggregating in this area were small (<4 m TL) males. Individuals were identified using photographs of distinctive scars and spot and stripe patterns on the sides of the animals. Of these, 65% had scarring that was attributable to boat or propeller strikes. Most of the whale sharks we encountered were feeding on dense accumulations of plankton in shallow water just off (10–200 m) the shoreline. This food source may account for the aggregation of sharks in this area. One 3 m male shark was tagged with an ARGOS (Splash) satellite tag for 9 d. During this time the shark traversed to the shoreline on the opposite side of the Gulf (a distance of 14 km) and then returned to the Arta Bay area before retracing his path to the other shore. The shark spent most of the daylight hours at the surface, while at night dives were more frequent, deeper and for longer durations.

Feeding anatomy,filter-feeding rate,and diet of whale sharks Rhincodon typus during surface ram filter feeding off the Yucatan Peninsula,Mexico

The feeding anatomy, behavior and diet of the whale shark Rhincodon typus were studied off Cabo Catoche, Yucatan Peninsula, Mexico. The filtering apparatus is composed of 20 unique filtering pads that completely occlude the pharyngeal cavity. A reticulated mesh lies on the proximal surface of the pads, with openings averaging 1.2 mm in diameter. Superficial to this, a series of primary and secondary cartilaginous vanes support the pads and direct the water across the primary gill filaments. During surface ram filter feeding, sharks swam at an average velocity of 1.1 m/s with 85% of the open mouth below the water's surface. Sharks on average spent approximately 7.5 h/day feeding at the surface on dense plankton dominated by sergestids, calanoid copepods, chaetognaths and fish larvae. Based on calculated flow speed and underwater mouth area, it was estimated that a whale shark of 443 cm total length (TL) filters 326 m³/h, and a 622 cm TL shark 614 m³/h. With an average plankton biomass of 4.5 g/m³ at the feeding site, the two sizes of sharks on average would ingest 1467 and 2763 g of plankton per hour, and their daily ration would be approximately 14,931 and 28,121 kJ, respectively. These values are consistent with independently derived feeding rations of captive, growing whale sharks in an aquarium. A feeding mechanism utilizing cross-flow filtration of plankton is described, allowing the sharks to ingest plankton that is smaller than the mesh while reducing clogging of the filtering apparatus.     

Oceanographic and atmospheric phenomena influence the abundance of whale sharks at Ningaloo Reef, Western Australia

Seasonal observations of whale shark abundance recorded by ecotourist operators at Ningaloo Reef, Western Australia from 1999 to 2004 were compared with weekly regional and global oceanographic and atmospheric variables, including average sea surface temperatures, along-shelf wind shear, sea level and the Southern Oscillation Index (SOI). Estimates of these physical variables were derived from either ground-based data or from remote sensing instruments. A generalised linear mixed-effects modelling (GLMM) approach with random sampling and model simulation was used to determine the relationships between the number of whale sharks and all model variants of the environmental parameters, using information-theoretic weights of evidence to rank models. SOI and wind shear had the most support for explaining the deviance in weekly whale shark abundance at Ningaloo Reef during a season. The SOI and wind shear variables positively influenced whale shark abundance such that more sharks were sighted when the Southern Oscillation was stronger and along-shelf winds were increasingly prevalent. This may reflect changes in the strength of oceanographic processes such as the Leeuwin Current (in response to the Southern Oscillation) and wind/current driven upwelling which may affect the abundance of whale sharks transported to the region and/or the availability of their prey by driving productivity changes.